This is what one vet said to use for persistent ear infections:

Dennis W. Thomas, DVM - "As a pet owner, it is important to know that persistent or recurring ear infections in a dog commonly are manifestations of other concurring disease processes. Allergies and hypothyroidism are good examples. Systemic allergies often cause dermatitis and itching in the ear canal, which predispose self-trauma and subsequent ear infection. Hypothyroidism may manifest itself as increased thickness of the skin and continual release of exudate in the ear canal, which favors an environment for otitis externa. It is not wise to use cotton tip applicators in the ear canal. This can be quite painful to the dog. Also, I don't recommend using alcohol in a dog's ear. Alcohol will dry the ear, but if there is inflammation or small scratches in the canal, the alcohol will burn and cause tremendous pain and further inflammation. If you must use a product from the shelf at home, use white vinegar diluted 50:50 with water. "

Some people, after that, put in a couple of drops of Grapefruit Seed Extract (diluted in a little water) because it kills all sorts of bacteria, fungus and yeast and is safe for pets. Health food stores sell it.